Fairfax Science Columnist

So, it's official at last: NASA's Voyager 1 spacecraft is the first man-made object to have left the solar system for the dark and mysterious realm of interstellar space.

Scientists are excited. "Voyager 1's departure is an incredible achievement in the context of space exploration," director of the Australian Astronomical Observatory Warrick Couch said. "It has covered about 19 billion kilometres since its launch, due largely to the sling-shot action of other planets."

"Not even NASA expected the craft to withstand the harsh environment of space for 36 years, much less bring fascinating results back of the kind of environment that exists beyond our solar system," Melbourne University astrophysicist Alan Duffy said.

The spacecraft is 120 times further away from the sun than is the Earth and it moves at about 17 kilometres a second.

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But amid the hullabaloo the vexed question remains: how big is the solar system, which includes everything that orbits the sun, from planets to tiny bits of icy debris? The controversy has raged since Galileo Galilei's telescope revealed a world beyond the then known world. In a sense, the solar system has been expanding in size - at least in terms of human consciousness - as astronomical technologies have improved.

Ultimately, the solar system's size depends on where the boundary is set. Some astronomers have suggested the outermost planet's orbit forms the limit. Since the demotion of Pluto, that has been Neptune. By that definition, the solar system would be about 9 billion kilometres across.

Others say it is bounded by the outer edge of the heliosphere, where the sun's sphere of influence, through the solar wind, peters out. That gives a diameter of more than 27 billion kilometres.

And other scientists say it might extend for more than two light-years to the outer edge of the Oort Cloud, a huge spherical collection of icy debris believed to have been ejected by gravitational interactions with the giant planets.

The cloud supposedly surrounds the sun and planets, but where it ends is uncertain. If it does exist, and is considered part of the solar system, that would yield a diameter of perhaps more than four light-years or roughly 40 trillion kilometres.

"There is some disagreement about the outer edge of the Oort Cloud, but most estimates suggest it is around 50,000 to 100,000 astronomical units away. In other words, one or two light years from the sun," CSIRO astrophysicist Kurt Liffman said. "By comparison, Voyager 1 is about 120 astronomical units off. In my view, Voyager 1 has only reached the outer edge of the heliosphere."

Dr Duffy said the true boundary probably lies somewhere in the region where the sun's gravity finally loses its influence. "This is many hundreds of times further out again, a dark and frozen zone filled with the leftover debris of early processes that formed the planets.''

With only 10 years nuclear-powered battery power left Voyager 1 will probably have shut down before reaching this region. "But it will sail on, finally approaching our neighbouring stars in 40,000 years' time," Dr Duffy said.